DEVX-155: fix: start local dockerd instead of using low-space inner dockerd

When no socket has sufficient space, don't fall back to the low-space
inner dockerd (which will fail on image pulls). Instead, kill the inner
dockerd, clean up its data root to free space, and start a local
dockerd on /dev/shm with vfs storage driver.

Also adds pkill of the inner dockerd and cleanup of its data root
(overlay2, image, volumes, containers) before starting the local
dockerd, to free up the 2.4GB used by the inner dockerd's data.

Generated with [Devin](https://devin.ai)

Co-Authored-By: Devin <158243242+devin-ai-integration[bot]@users.noreply.github.com>

tests/unit/test_start_docker.py

@@ -170,6 +170,7 @@ class TestStartDockerDaemon:
     @patch("devx.molecule.start_docker.is_docker_ready", return_value=True)
     @patch("devx.molecule.start_docker.os.path.exists", side_effect=_exists_map({DOCKER_SOCK}))
     @patch("devx.molecule.start_docker.glob.glob", return_value=[])
+    @patch("devx.molecule.start_docker.subprocess.run")
     @patch("devx.molecule.start_docker.subprocess.Popen")
     @patch("devx.molecule.start_docker.tempfile.NamedTemporaryFile")
     @patch("devx.molecule.start_docker.time.sleep")
@@ -178,6 +179,7 @@ class TestStartDockerDaemon:
         mock_sleep: MagicMock,
         mock_ntf: MagicMock,
         mock_popen: MagicMock,
+        mock_run: MagicMock,
         mock_glob: MagicMock,
         mock_exists: MagicMock,
         mock_ready: MagicMock,
@@ -196,6 +198,7 @@ class TestStartDockerDaemon:
     @patch("devx.molecule.start_docker.is_docker_ready")
     @patch("devx.molecule.start_docker.os.path.exists", side_effect=_exists_map({DOCKER_SOCK}))
     @patch("devx.molecule.start_docker.glob.glob", return_value=[])
+    @patch("devx.molecule.start_docker.subprocess.run")
     @patch("devx.molecule.start_docker.subprocess.Popen")
     @patch("devx.molecule.start_docker.tempfile.NamedTemporaryFile")
     @patch("devx.molecule.start_docker.time.sleep")
@@ -204,6 +207,7 @@ class TestStartDockerDaemon:
         mock_sleep: MagicMock,
         mock_ntf: MagicMock,
         mock_popen: MagicMock,
+        mock_run: MagicMock,
         mock_glob: MagicMock,
         mock_exists: MagicMock,
         mock_ready: MagicMock,
@@ -300,18 +304,28 @@ class TestStartDockerDaemon:
     @patch("devx.molecule.start_docker.is_docker_ready")
     @patch("devx.molecule.start_docker.os.path.exists", side_effect=_exists_map({DOCKER_SOCK}))
     @patch("devx.molecule.start_docker.glob.glob", return_value=[])
-    def test_all_sockets_low_space_falls_back(
+    @patch("devx.molecule.start_docker.subprocess.run")
+    @patch("devx.molecule.start_docker.subprocess.Popen")
+    @patch("devx.molecule.start_docker.tempfile.NamedTemporaryFile")
+    @patch("devx.molecule.start_docker.time.sleep")
+    def test_all_sockets_low_space_starts_local(
         self,
+        mock_sleep: MagicMock,
+        mock_ntf: MagicMock,
+        mock_popen: MagicMock,
+        mock_run: MagicMock,
         mock_glob: MagicMock,
         mock_exists: MagicMock,
         mock_ready: MagicMock,
         mock_free: MagicMock,
         mock_diag: MagicMock,
     ) -> None:
-        """Should fall back to a low-space socket if no better option exists."""
-        mock_ready.side_effect = [True, True, True, True]
-        mock_free.side_effect = [5 * 1024**3, 5 * 1024**3, 5 * 1024**3, 5 * 1024**3]
+        """Should start local dockerd if all sockets have low space."""
+        mock_ntf.return_value = MagicMock(name="/tmp/dockerd.log")
+        mock_ready.side_effect = [True, False, False, False, False, True]
+        mock_free.return_value = 5 * 1024**3
         assert start_docker_daemon(timeout=5) is True
+        mock_popen.assert_called_once()
 
     @patch("devx.molecule.start_docker.glob.glob", return_value=[])
     @patch("devx.molecule.start_docker._diagnose_socket")
@@ -370,7 +384,7 @@ class TestStartDockerDaemon:
         with patch("builtins.open", mock_open(read_data="dockerd error log")):
             assert start_docker_daemon(timeout=3) is False
         mock_popen.assert_called_once()
-        assert mock_sleep.call_count == 3
+        assert mock_sleep.call_count == 4  # 1 after pkill + 3 timeout retries
 
     @patch("devx.molecule.start_docker.glob.glob", return_value=[])
     @patch("devx.molecule.start_docker._diagnose_socket")
@@ -424,7 +438,7 @@ class TestStartDockerDaemon:
         mock_ready.side_effect = [False, False, True]
         assert start_docker_daemon(timeout=5) is True
         assert mock_popen.call_count == 1
-        assert mock_sleep.call_count == 2
+        assert mock_sleep.call_count == 3  # 1 after pkill + 2 loop retries
 
     @patch("devx.molecule.start_docker._diagnose_socket")
     @patch("devx.molecule.start_docker._get_docker_free_bytes", return_value=100 * 1024**3)